1. Field of the Invention
This invention relates to an improved, energy efficient and environmentally attractive process for beneficiating and dewatering high water content carbonaceous materials by a "dry" carbonization process to yield a superior fuel or intermediate product for further conversion.
2. Description of the Prior Art
The declining availability of petroleum based products as energy resources has spurred research efforts to provide alternative fuel resources. Most organic carbonaceous materials can be converted to a form which can be utilized as fuel, but conversion efficiencies and overall process efficiencies are undesirably low. The development of viable alternative energy sources depends to a large extent upon the development of energy efficient conversion processes.
Abundant and inexpensive organic carbonaceous raw materials, such as peat, lignite, kelp, mulch, and other biomass materials may be carbonized and dewatered to provide fuel. These materials, in their naturally occurring states, contain substantial amounts of water. For example, raw peat may comprise as much as 90 weight percent water. The bulk of the water must be removed from these organic carbonaceous materials before they can be utilized as a source of energy. Several water removal methods are known to the art.
Water removal has been accomplished by mechanical means, such as roller and belt presses or screening devices. Water removal by mechanical means alone, however, is inadequate. Peat, for example, may be mechanically dewatered to reduce the moisture content to about 65 to 75 weight percent. To achieve the desired final moisture content, usually 50 weight percent moisture or less, mechanically dewatered organic carbonaceous material ordinarily must be thermally dried. Thermal drying requires substantial energy inputs which render the process of mechanical dewatering in combination with thermal drying economically unattractive.
Solar energy may be utilized to air dry organic carbonaceous material to achieve a moisture content of less than 50 weight percent. This dewatering procedure is energy efficient, but its success is wholly weather dependent, and it requires continuous harvesting of large land areas. Utilization of solar energy to air dry organic carbonaceous material is not presently commercially viable.
Various pretreatment methods have been developed to facilitate the mechanical dewatering of organic carbonaceous materials to achieve a lower final moisture content. The pretreatment processes can be categorized as mechanical, biological and thermal. Mechanical pretreatment techniques are designed to remove colloidal matter prior to mechanical dewatering. Biological pretreatment effects limited biodegradation of organic carbonaceous materials and may also improve dewaterability, but the results of biological pretreatment are not well documented.
Thermal pretreatment methods which facilitate mechanical dewatering are generally referred to as wet carbonization processes. Typically, wet carbonization entails heating the organic carbonaceous material to temperatures of about 300.degree. to about 1000.degree. F. at pressures of about 300 to about 2000 psi, or even higher to prevent vaporization of water, for residence times of about one hour or less. Chemical decarboxylation and dehydration reactions occur during wet carbonization that permit more effective mechanical dewatering and enhance the heating value of the dewatered solids. Several wet carbonization processes are reviewed in Mensinger, Michael C., "Wet Carbonization of Peat; State of the Art Review", Peat as an Energy Alternative Symposium Papers, published by the Institute of Gas Technology, Chicago, Ill., September 1981, pp. 249-280. All of the systems reviewed in this paper employ water slurry transport systems imposing a large heat transfer load to the process and extraction of desired carbonaceous materials from the feed to the water slurry.
U.S. Pat. Nos. 2,624,712 and 4,268,417 teach processes for converting organic carbonaceous materials to activated carbon. The U.S. Pat. No. 2,624,712 patent teaches conversion of tars and pitches by solvent extraction and the U.S. Pat. No. 4,268,417 patent teaches conversion of coal using a water slurry followed by physical separation prior to heat treatment.
Pneumatic transport of pulverized fuel is known to the art, and is disclosed in U.S. Pat. No. 1,203,703. U.S. Pat. No. 4,153,427 teaches a method for feeding coal to a gasifier wherein an aqueous coal slurry is pressurized and then dried by contacting the slurry with superheated steam in an entrained bed drier prior to feeding coal particles to the gasifier. U.S. Pat. No. 4,363,636 discloses a method for converting raw bagasse to a usable fuel energy source by drying with hot flue gases, classifying by particle size, and densifying to pellet form to provide usable fuel.